Thermoset particles with enhanced crosslinking, processing for their production, and their use in oil and natural gas drilling applications

ABSTRACT

A method for fracture stimulation of a subterranean formation having a wellbore. The method comprise a series of steps. A slurry is injected into the wellbore at sufficiently high rates and pressures such that the formation fails and fractures to accept the slurry. The slurry comprises a fluid and a proppant, wherein said proppant comprises a styrene-ethylvinylbenzene-divinylbenzene terpolymer composition having a substantially cured polymer network, wherein said composition lacks rigid fillers or nanofillers. The proppant is emplaced within the fracture network in a packed mass or a partial monolayer of the proppant within the fracture, wherein the packed mass or partial monolayer props open the fracture; thereby allowing produced gases, fluids, or mixtures thereof, to flow towards the wellbore.

The present application is a divisional of prior U.S. application Ser.No. 11/451,697, filed 13 Jun. 2006, which claims priority from U.S.Provisional Application Ser. No. 60/689,899, filed 13 Jun. 2005, (nowexpired), each of which are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to lightweight thermoset polymerparticles, to processes for the manufacture of such particles, and toapplications of such particles. It is possible to use a wide range ofthermoset polymers as the main constituents of the particles of theinvention, and to produce said particles by means of a wide range offabrication techniques. Without reducing the generality of theinvention, in its currently preferred embodiments, the thermoset polymerconsists of a terpolymer of styrene, ethyvinylbenzene anddivinylbenzene; suspension polymerization is performed to prepare theparticles, and post-polymerization heat treatment is performed with theparticles placed in an unreactive gaseous environment with nitrogen asthe preferred unreactive gas to further advance the curing of thethermoset polymer. When executed in the manner taught by thisdisclosure, many properties of both the individual particles andpackings of the particles can be improved by the practice of theinvention. The particles exhibit enhanced stiffness, strength, heatresistance, and resistance to aggressive environments; as well as theimproved retention of high conductivity of liquids and gases throughpackings of the particles in aggressive environments under highcompressive loads at elevated temperatures. The thermoset polymerparticles of the invention can be used in many applications. Theseapplications include, but are not limited to, the construction,drilling, completion and/or fracture stimulation of oil and natural gaswells; for example, as a proppant partial monolayer, a proppant pack, anintegral component of a gravel pack completion, a ball bearing, a solidlubricant, a drilling mud constituent, and/or a cement additive.

BACKGROUND

The background of the invention can be described most clearly, and hencethe invention can be taught most effectively, by subdividing thissection in three subsections. The first subsection will provide somegeneral background regarding the role of crosslinked (and especiallystiff and strong thermoset) particles in the field of the invention. Thesecond subsection will describe the prior art that has been taught inthe patent literature. The third subsection will provide additionalrelevant background information selected from the vast scientificliterature on polymer materials science and chemistry, to furtherfacilitate the teaching of the invention.

A. General Background

Crosslinked polymer (and especially stiff and strong thermoset)particles are used in many applications requiring high stiffness, highmechanical strength, high temperature resistance, and/or high resistanceto aggressive environments. Crosslinked polymer particles can beprepared by reacting monomers or oligomers possessing three or morereactive chemical functionalities, as well as by reacting mixtures ofmonomers and/or oligomers at least one ingredient of which possessesthree or more reactive chemical functionalities.

The intrinsic advantages of crosslinked polymer particles over polymerparticles lacking a network consisting of covalent chemical bonds insuch applications become especially obvious if an acceptable level ofperformance must be maintained for a prolonged period (such as manyyears, or in some applications even several decades) under the combinedeffects of mechanical deformation, heat, and/or severe environmentalinsults. For example, many high-performance thermoplastic polymers,which have excellent mechanical properties and which are hence usedsuccessfully under a variety of conditions, are unsuitable forapplications where they must maintain their good mechanical propertiesfor many years in the presence of heat and/or chemicals, because theyconsist of assemblies of individual polymer chains. Over time, thedeformation of such assemblies of individual polymer chains at anelevated temperature can cause unacceptable amounts of creep, andfurthermore solvents and/or aggressive chemicals present in theenvironment can gradually diffuse into them and degrade theirperformance severely (and in some cases even dissolve them). Bycontrast, the presence of a well-formed continuous network of covalentbonds restrains the molecules, thus helping retain an acceptable levelof performance under severe use conditions over a much longer timeperiod.

Oil and natural gas well construction activities, including drilling,completion and stimulation applications (such as proppants, gravel packcomponents, ball bearings, solid lubricants, drilling mud constituents,and/or cement additives), require the use of particulate materials, inmost instances preferably of as nearly spherical a shape as possible.These (preferably substantially spherical) particles must generally bemade from materials that have excellent mechanical properties. Themechanical properties of greatest interest in most such applications arestiffness (resistance to deformation) and strength under compressiveloads, combined with sufficient “toughness” to avoid the brittlefracture of the particles into small pieces commonly known as “fines”.In addition, the particles must have excellent heat resistance in orderto be able to withstand the combination of high compressive load andhigh temperature that normally becomes increasingly more severe as onedrills deeper. In other words, particles that are intended for usedeeper in a well must be able to withstand not only the higheroverburden load resulting from the greater depth, but also the highertemperature that accompanies that higher overburden load as a result ofthe nature of geothermal gradients. Finally, these materials must beable to withstand the effects of the severe environmental insults(resulting from the presence of a variety of hydrocarbon and possiblysolvent molecules as well as water, at simultaneously elevatedtemperatures and compressive loads) that the particles will encounterdeep in an oil or natural gas well. The need for relatively lightweighthigh performance materials for use in these particulate components inapplications related to the construction, drilling, completion and/orfracture stimulation of oil and natural gas wells thus becomes obvious.Consequently, while such uses constitute only a small fraction of theapplications of stiff and strong materials, they provide fertileterritory for the development of new or improved materials andmanufacturing processes for the fabrication of such materials.

We will focus much of the remaining discussion of the background of theinvention on the use of particulate materials as proppants. One keymeasure of end use performance of proppants is the retention of highconductivity of liquids and gases through packings of the particles inaggressive environments under high compressive loads at elevatedtemperatures.

The use of stiff and strong solid proppants has a long history in theoil and natural gas industry. Throughout most of this history, particlesmade from polymeric materials (including crosslinked polymers) have beenconsidered to be unsuitable for use by themselves as proppants. Thereason for this prejudice is the perception that polymers are toodeformable, as well as lacking in the ability to withstand thecombination of elevated compressive loads, temperatures and aggressiveenvironments that are commonly encountered in oil and natural gas wells.Consequently, work on proppant material development has focused mainlyon sands, on ceramics, and on sands and ceramics coated by crosslinkedpolymers to improve some aspects of their performance. This situationhas prevailed despite the fact that most polymers have densities thatare much closer to that of water so that in particulate form they can betransported much more readily into a fracture by low-density fracturingor carrier fluids such as unviscosified water.

Nonetheless, the obvious practical advantages [see a review by Edgeman(2004)] of developing the ability to use lightweight particles thatpossess almost neutral buoyancy relative to water have stimulated aconsiderable amount of work over the years. However, as will be seenfrom the review of the prior art provided below, progress in this fieldof invention has been very slow as a result of the many technicalchallenges that exist to the successful development of cost-effectivelightweight particles that possess sufficient stiffness, strength andheat resistance.

B. Prior Art

The prior art can be described most clearly, and hence the invention canbe placed in the proper context most effectively, by subdividing thissection into two subsections. The first subsection will describe priorart related to the development of “as-polymerized” thermoset polymerparticles. The second subsection will describe prior art related to thedevelopment of thermoset polymer particles that are subjected topost-polymerization heat treatment.

1. “As-Polymerized” Thermoset Polymer Particles

As discussed above, particles made from polymeric materials havehistorically been considered to be unsuitable for use by themselves asproppants. Consequently, their past uses in proppant materials havefocused mainly on their placement as coatings on sands and ceramics, inorder to improve some aspects of the performance of the sand and ceramicproppants.

Significant progress was made in the use of crosslinked polymericparticles themselves as constituents of proppant formulations in priorart taught by Rickards, et al. (U.S. Pat. Nos. 6,059,034; 6,330,916).However, these inventors still did not consider or describe thepolymeric particles as proppants. Their invention only related to theuse of the polymer particles in blends with particles of moreconventional proppants such as sands or ceramics. They taught that thesand or ceramic particles are the proppant particles, and that the“deformable particulate material” consisting of polymer particles mainlyserves to improve the fracture conductivity, reduce the generation offines and/or reduce proppant flowback relative to the unblended sand orceramic proppants. Thus while their invention differs significantly fromthe prior art in the sense that the polymer is used in particulate formrather than being used as a coating, it shares with the prior art thelimitation that the polymer still serves merely as a modifier improvingthe performance of a sand or ceramic proppant rather than beingconsidered for use as a proppant in its own right.

Bienvenu (U.S. Pat. No. 5,531,274) disclosed progress towards thedevelopment of lightweight proppants consisting of high-strengthcrosslinked polymeric particles for use in hydraulic fracturingapplications. However, embodiments of this prior art, based on the useof styrene-divinylbenzene (S-DVB) copolymer beads manufactured by usingconventional fabrication technology and purchased from a commercialsupplier, failed to provide an acceptable balance of performance andprice. They cost far more than the test standard (Jordan sand) whilebeing outperformed by Jordan sand in terms of the liquid conductivityand liquid permeability characteristics of their packings measuredaccording to the industry-standard API RP 61 testing procedure. [Thisprocedure is described by the American Petroleum Institute in itspublication titled “Recommended Practices for Evaluating Short TermProppant Pack Conductivity” (first edition, Oct. 1, 1989).] The need touse a very large amount of an expensive crosslinker (50 to 80% by weightof DVB) in order to obtain reasonable performance (not too inferior tothat of Jordan Sand) was a key factor in the higher cost thataccompanied the lower performance.

The most advanced prior art in stiff and strong crosslinked“as-polymerized” polymer particle technologies for use in applicationsin oil and natural gas drilling was developed by Albright (U.S. Pat. No.6,248,838) who taught the concept of a “rigid chain entanglementcrosslinked polymer”. In summary, the reactive formulation and theprocessing conditions were modified to achieve “rapid ratepolymerization”. While not improving the extent of covalent crosslinkingrelative to conventional isothermal polymerization, rapid ratepolymerization results in the “trapping” of an unusually large number ofphysical entanglements in the polymer. These additional entanglementscan result in a major improvement of many properties. For example, theliquid conductivities of packings of S-DVB copolymer beads withw_(DVB)=0.2 synthesized via rapid rate polymerization are comparable tothose that were found by Bienvenu (U.S. Pat. No. 5,531,274) for packingsof conventionally produced S-DVB beads at the much higher DVB level ofw_(DVB)=0.5. Albright (U.S. Pat. No. 6,248,838) thus provided the keytechnical breakthrough that enabled the development of the firstgeneration of crosslinked polymer beads possessing sufficientlyattractive combinations of performance and price characteristics toresult in their commercial use in their own right as solid polymericproppants.

2. Heat-Treated Thermoset Polymer Particles

There is no prior art that relates to the development of heat-treatedthermoset polymer particles that have not been reinforced by rigidfillers or by nanofillers for use in oil and natural gas wellconstruction applications. One needs to look into another field oftechnology to find prior art of some relevance related to such“unfilled” heat-treated thermoset polymer particles. Nishimori, et. al.(JP1992-22230) focused on the development of particles for use in liquidcrystal display panels. They taught the use of post-polymerization heattreatment to increase the compressive elastic modulus of S-DVB particlesat room temperature. They only claimed compositions polymerized fromreactive monomer mixtures containing 20% or more by weight of DVB orother crosslinkable monomer(s) prior to the heat treatment. They statedexplicitly that improvements obtained with lower weight fractions of thecrosslinkable monomer(s) were insufficient and that hence suchcompositions were excluded from the scope of their patent.

C. Scientific Literature

The development of thermoset polymers requires the consideration of avast and multidisciplinary range of polymer materials science andchemistry challenges. It is essential to convey these challenges in thecontext of the fundamental scientific literature.

Bicerano (2002) provides a broad overview of polymer materials sciencethat can be used as a general reference for most aspects of thefollowing discussion. Additional references will also be provided below,to other publications which treat specific issues in greater detail thanwhat could be accommodated in Bicerano (2002).

1. Selected Fundamental Aspects of the Curing of Crosslinked Polymers

It is essential, first, to review some fundamental aspects of the curingof crosslinked polymers, which are applicable to such polymersregardless of their form (particulate, coating, or bulk).

The properties of crosslinked polymers prepared by standardmanufacturing processes are often limited by the fact that suchprocesses typically result in incomplete curing. For example, in anisothermal polymerization process, as the glass transition temperature(T_(g)) of the growing polymer network increases, it may reach thepolymerization temperature while the reaction is still in progress. Ifthis happens, then the molecular motions slow down significantly so thatfurther curing also slows down significantly. Incomplete curing yields apolymer network that is less densely crosslinked than the theoreticallimit expected from the functionalities and relative amounts of thestarting reactants. For example, a mixture of monomers might contain 80%DVB by weight as a crosslinker but the final extent of crosslinking thatis attained may not be much greater than what was attained with a muchsmaller percentage of DVB. This situation results in lower stiffness,lower strength, lower heat resistance, and lower environmentalresistance than the thermoset is capable of manifesting when it is fullycured and thus maximally crosslinked.

When the results of the first scan and the second scan of S-DVB beadscontaining various weight fractions of DVB (w_(DVB)), obtained byDifferential Scanning calorimetry (DSC), as reported by Bicerano, et al.(1996) (see FIG. 1) are compared, it becomes clear that the lowperformance and high cost of the “as purchased” S-DVB beads utilized byBienvenu (U.S. Pat. No. 5,531,274) are related to incomplete curing.This incomplete curing results in the ineffective utilization of DVB asa crosslinker and thus in the incomplete development of the crosslinkednetwork. In summary, Bicerano, et al. (1996), showed that the T_(g) oftypical “as-polymerized” S-DVB copolymers, as measured by the first DSCscan, increased only slowly with increasing w_(DVB), and furthermorethat the rate of further increase of T_(g) slowed down drastically forw_(DVB)>0.08. By contrast, in the second DSC scan (performed on S-DVBspecimens whose curing had been driven much closer to completion as aresult of the temperature ramp that had been applied during the firstscan), T_(g) grew much more rapidly with w_(DVB) over the entire rangeof up to w_(DVB)=0.2458 that was studied. The more extensively curedsamples resulting from the thermal history imposed by the first DSC scancan, thus, be considered to provide much closer approximations to theideal theoretical limit of a “fully cured” polymer network.

2. Effects of Heat Treatment on Key Properties of Thermoset Polymers

a. Maximum Possible Use Temperature

As was illustrated by Bicerano, et al. (1996) for S-DVB copolymers withw_(DVB) of up to 0.2458, enhancing the state of cure of a thermosetpolymer network can increase T_(g) very significantly relative to theT_(g) of the “as-polymerized” material. In practice, the heat distortiontemperature (HDT) is used most often as a practical indicator of thesoftening temperature of a polymer under load. As was shown by Takemori(1979), a systematic understanding of the HDT is possible through itsdirect correlation with the temperature dependences of the tensile (orequivalently, compressive) and shear elastic moduli. For amorphouspolymers, the precipitous decrease of these elastic moduli as T_(g) isapproached from below renders the HDT well-defined, reproducible, andpredictable. HDT is thus closely related to (and usually slightly lowerthan) T_(g) for amorphous polymers, so that it can be increasedsignificantly by increasing T_(s) significantly.

The HDT decreases gradually with increasing magnitude of the load usedin its measurement. For example, for general-purpose polystyrene (whichhas T_(g)=100° C.), HDT=95° C. under a load of 0.46 MPa and HDT=85° C.under a load of 1.82 MPa are typical values. However, the compressiveloads deep in an oil well or natural gas well are normally far higherthan the standard loads (0.46 MPa and 1.82 MPa) used in measuring theHDT. Consequently, amorphous thermoset polymer particles can be expectedto begin to deform significantly at a lower temperature than the HDT ofthe polymer measured under the standard high load of 1.82 MPa. Thisdeformation will cause a decrease in the conductivities of liquids andgases through the propped fracture, and hence in the loss ofeffectiveness as a proppant, at a somewhat lower temperature than theHDT value of the polymer measured under the standard load of 1.82 MPa.

b. Mechanical Properties

As was discussed earlier, Nishimori, et. al. (JP1992-22230) used heattreatment to increase the compressive elastic modulus of their S-DVBparticles (intended for use in liquid crystal display panels)significantly at room temperature (and hence far below T_(g)).Deformability under a compressive load is inversely proportional to thecompressive elastic modulus. It is, therefore, important to considerwhether one may also anticipate major benefits from heat treatment interms of the reduction of the deformability of thermoset polymerparticles intended for oil and natural gas drilling applications, whenthese particles are used in subterranean environments where thetemperature is far below the T_(g) of the particles. As explained below,the enhancement of curing via post-polymerization heat treatment isgenerally expected to have a smaller effect on the compressive elasticmodulus (and hence on the proppant performance) of thermoset polymerparticles when used in oil and natural gas drilling applications attemperatures far below their T_(g).

Nishimori, et. al. (JP1992-22230) used very large amounts of DVB(w_(DVB)>>0.2). By contrast, in general, much smaller amounts of DVB(w_(DVB)≦0.2) must be used for economic reasons in the “lower value” oiland natural gas drilling applications. The elastic moduli of a polymerat temperatures far below T_(g) are determined primarily by deformationsthat are of a rather local nature and hence on a short length scale.Some enhancement of the crosslink density via further curing (when thenetwork junctions created by the crosslinks are far away from each otherto begin with) will hence not normally have nearly as large an effect onthe elastic moduli as when the network junctions are very close to eachother to begin with and then are brought even closer by the enhancementof curing via heat treatment. Consequently, while the compressiveelastic modulus can be expected to increase significantly upon heattreatment when w_(DVB) is very large, any such effect will normally beless pronounced at low values of w_(DVB). In summary, it can thusgenerally be expected that the enhancement of the compressive elasticmodulus at temperatures far below T_(g) will probably be small for thetypes of formulations that are most likely to be used in the synthesisof thermoset polymer particles for oil and natural gas drillingapplications.

SUMMARY OF THE INVENTION

The present invention involves a novel approach towards the practicaldevelopment of stiff, strong, tough, heat resistant, and environmentallyresistant ultralightweight particles, for use in the construction,drilling, completion and/or fracture stimulation of oil and natural gaswells.

The disclosure is summarized below in three key aspects: (A)Compositions of Matter (thermoset particles that exhibit improvedproperties compared with prior art), (B) Processes (methods formanufacture of the compositions of matter), and (C) Applications(utilization of the compositions of matter in the construction,drilling, completion and/or fracture stimulation of oil and natural gaswells).

The disclosure describes lightweight thermoset polymer particles whoseproperties are improved relative to prior art. The particles targetedfor development include, but are not limited to, terpolymers of styrene,ethyvinylbenzene and divinylbenzene. The particles exhibit any one orany combination of the following properties: enhanced stiffness,strength, heat resistance, and/or resistance to aggressive environments;and/or improved retention of high conductivity of liquids and/or gasesthrough packings of the particles when the packings are placed inpotentially aggressive environments under high compressive loads atelevated temperatures.

The disclosure also describes processes that can be used to manufacturethe particles. The fabrication processes targeted for developmentinclude, but are not limited to, suspension polymerization to preparethe “as-polymerized” particles, and post-polymerization process(es) tofurther advance the curing of the polymer. The post-polymerizationprocess(es) may optionally comprise heat treatment. The particles duringthe heat treatment are placed in an unreactive gaseous environment withnitrogen as the preferred unreactive gas.

The disclosure finally describes the use of the particles in practicalapplications. The targeted applications include, but are not limited to,the construction, drilling, completion and/or fracture stimulation ofoil and natural gas wells; for example, as a proppant partial monolayer,a proppant pack, an integral component of a gravel pack completion, aball bearing, a solid lubricant, a drilling mud constituent, and/or acement additive.

A. Compositions of Matter

The compositions of matter of the present invention are thermosetpolymer particles. Any additional formulation component(s) familiar tothose skilled in the art can also be used during the preparation of theparticles; such as initiators, catalysts, inhibitors, dispersants,stabilizers, rheology modifiers, buffers, antioxidants, defoamers,impact modifiers, plasticizers, pigments, flame retardants, smokeretardants, or mixtures thereof. Some of the additional component(s) mayalso become either partially or completely incorporated into theparticles in some embodiments of the invention. However, the onlyrequired component of the particles is a thermoset polymer.

Any rigid thermoset polymer may be used as the polymer of the presentinvention. Rigid thermoset polymers are, in general, amorphous polymerswhere covalent crosslinks provide a three-dimensional network. However,unlike thermoset elastomers (often referred to as “rubbers”) which alsopossess a three-dimensional network of covalent crosslinks, the rigidthermosets are, by definition, “stiff”. In other words, they have highelastic moduli at “room temperature” (25° C.), and often up to muchhigher temperatures, because their combinations of chain segmentstiffness and crosslink density result in a high glass transitiontemperature.

Some examples of rigid thermoset polymers that can be used as materialsof the invention will be provided below. It is to be understood thatthese examples are being provided without reducing the generality of theinvention, merely to facilitate the teaching of the invention.

Commonly used rigid thermoset polymers include, but are not limited to,crosslinked epoxies, epoxy vinyl esters, polyesters, phenolics,melamine-based resins, polyurethanes, and polyureas. Rigid thermosetpolymers that are used less often because of their high cost despitetheir exceptional performance include, but are not limited to,crosslinked polyimides. These various types of polymers can, indifferent embodiments of the invention, be prepared by starting eitherfrom their monomers, or from oligomers that are often referred to as“prepolymers”, or from suitable mixtures of monomers and oligomers.

Many additional types of rigid thermoset polymers can also be used inparticles of the invention, and are all within the scope of theinvention. Such polymers include, but are not limited to, variousfamilies of crosslinked copolymers prepared most often by thepolymerization of vinylic monomers, of vinylidene monomers, or ofmixtures thereof.

The “vinyl fragment” is commonly defined as the CH₂═CH— fragment. So a“vinylic monomer” is a monomer of the general structure CH₂═CHR where Rcan be any one of a vast variety of molecular fragments or atoms (otherthan hydrogen). When a vinylic monomer CH₂═CHR reacts, it isincorporated into the polymer as the —CH₂—CHR— repeat unit. Among rigidthermosets built from vinylic monomers, the crosslinked styrenics andcrosslinked acrylics are especially familiar to workers in the field.Some other familiar types of vinylic monomers (among others) include theolefins, vinyl alcohols, vinyl esters, and vinyl halides.

The “vinylidene fragment” is commonly defined as the CH₂═CR″-fragment.So a “vinylidene monomer” is a monomer of the general structureCH₂═CR′R″ where R′ and R″ can each be any one of a vast variety ofmolecular fragments or atoms (other than hydrogen). When a vinylidenemonomer CH₂═CR′R″ reacts, it is incorporated into a polymer as the—CH₂—CR′R″-repeat unit. Among rigid thermosets built from vinylidenepolymers, the crosslinked alkyl acrylics [such as crosslinkedpoly(methyl methacrylate)] are especially familiar to workers in thefield. However, vinylidene monomers similar to each type of vinylmonomer (such as the styrenics, acrylates, olefins, vinyl alcohols,vinyl esters and vinyl halides, among others) can be prepared. Oneexample of particular interest in the context of styrenic monomers is□-methyl styrene, a vinylidene-type monomer that differs from styrene (avinyl-type monomer) by having a methyl (—CH₃) group serving as the R″fragment replacing the hydrogen atom attached to the □-carbon.

Thermosets based on vinylic monomers, on vinylidene monomers, or onmixtures thereof, are typically prepared by the reaction of a mixturecontaining one or more non-crosslinking (difunctional) monomer and oneor more crosslinking (three or higher functional) monomers. Allvariations in the choices of the non-crosslinking monomer(s), thecrosslinking monomers(s), and their relative amounts [subject solely tothe limitation that the quantity of the crosslinking monomer(s) must notbe less than 1% by weight], are within the scope of the invention.

Without reducing the generality of the invention, in its currentlypreferred embodiments, the thermoset polymer particles consist of aterpolymer of styrene (non-crosslinking), ethyvinylbenzene (alsonon-crosslinking), and divinylbenzene (crosslinking), with the weightfraction of divinylbenzene ranging from 3% to 35% by weight of thestarting monomer mixture.

B. Processes

If a suitable post-polymerization process step is applied to thermosetpolymer particles, in many cases the curing reaction will be drivenfurther towards completion so that T_(g) (and hence also the maximumpossible use temperature) will increase. This is the most commonlyobtained benefit of applying a post-polymerization process step. In someinstances, there may also be further benefits, such as an increase inthe compressive elastic modulus even at temperatures that are far belowT_(g), and an increase of such magnitude in the resistance to aggressiveenvironments as to enhance significantly the potential range ofapplications of the particles.

Processes that may be used to enhance the degree of curing of athermoset polymer include, but are not limited to, heat treatment (whichmay be combined with stirring, flow and/or sonication to enhance itseffectiveness), electron beam irradiation, and ultraviolet irradiation.FIG. 2 provides an idealized schematic illustration of the benefits ofimplementing such methods. We focused mainly on the use of heattreatment in order to increase the T_(g) of the thermoset polymer.

The processes that may be used for the fabrication of the thermosetpolymer particles of the invention comprise two major steps. The firststep is the formation of the particles by means of a polymerizationprocess. The second step is the use of an appropriate postcuring methodto advance the curing reaction and to thus obtain a thermoset polymernetwork that approaches the “fully cured” limit. Consequently, thissubsection will be further subdivided into two subsections, dealing withpolymerization and with postcure respectively.

1. Polymerization and Network Formation

Any method for the fabrication of thermoset polymer particles known tothose skilled in the art may be used to prepare embodiments of theparticles of the invention. Without reducing the generality of theinvention, our preferred method will be discussed below to facilitatethe teaching of the invention.

It is especially practical to prepare the particles by using methodsthat can produce the particles directly in the desired (usuallysubstantially spherical) shape during polymerization from the startingmonomers. (While it is a goal of this invention to create sphericalparticles, it is understood that it is exceedingly difficult as well asunnecessary to obtain perfectly spherical particles. Therefore,particles with minor deviations from a perfectly spherical shape areconsidered perfectly spherical for the purposes of this disclosure.)Suspension (droplet) polymerization is the most powerful methodavailable for accomplishing this objective.

Two main approaches exist to suspension polymerization. The firstapproach is isothermal polymerization which is the conventional approachthat has been practiced for many decades. The second approach is “rapidrate polymerization” as taught by Albright (U.S. Pat. No. 6,248,838)which is incorporated herein by reference in its entirety. Withoutreducing the generality of the invention, suspension polymerization asperformed via the rapid rate polymerization approach taught by Albright(U.S. Pat. No. 6,248,838) is used in the current preferred embodimentsof the invention.

2. Post-Polymerization Advancement of Curing and Network Formation

As was discussed earlier and illustrated in FIG. 1 with the data ofBicerano, et al. (1996), typical processes for the synthesis ofthermoset polymers may result in the formation of incompletely curednetworks, and may hence produce thermosets with lower glass transitiontemperatures and lower maximum use temperatures than is achievable withthe chosen formulation of reactants. Consequently, the use of apost-polymerization process step (or a sequence of such process steps)to advance the curing of a thermoset polymer particle of the inventionis an aspect of the invention. Suitable methods include, but are notlimited to, heat treatment (also known as “annealing”), electron beamirradiation, and ultraviolet irradiation.

Post-polymerization heat treatment is a very powerful method forimproving the properties and performance of S-DVB copolymers (as well asof many other types of thermoset polymers) by helping the polymernetwork approach its “full cure” limit. It is, in fact, the most easilyimplementable method for advancing the state of cure of S-DVB copolymerparticles. However, it is important to recognize that anotherpost-polymerization method (such as electron beam irradiation orultraviolet irradiation) may be the most readily implementable one foradvancing the state of cure of some other type of thermoset polymer. Theuse of any suitable method for advancing the curing of the thermosetpolymer that is being used as a particle of the present invention afterpolymerization is within the scope of the invention.

Without reducing the generality of the invention, among the suitablemethods, heat treatment is used as the post-polymerization method toenhance the curing of the thermoset polymer in the preferred embodimentsof the invention. Any desired thermal history can be imposed; such as,but not limited to, isothermal annealing at a fixed temperature;nonisothermal heat exposure with either a continuous or a step functiontemperature ramp; or any combination of continuous temperature ramps,step function temperature ramps, and/or periods of isothermal annealingat fixed temperatures. In practice, while there is great flexibility inthe choice of a thermal history, it must be selected carefully to drivethe curing reaction to the maximum final extent possible withoutinducing unacceptable levels of thermal degradation.

Any significant increase in T_(g) by means of improved curing willtranslate directly into an increase of comparable magnitude in thepractical softening temperature of the polymer particles under thecompressive load imposed by the subterranean environment. Consequently,a significant increase of the maximum possible use temperature of thethermoset polymer particles is the most common benefit of advancing theextent of curing by heat treatment.

A practical concern during the imposition of heat treatment is relatedto the amount of material that is being subjected to heat treatmentsimultaneously. For example, very small amounts of material can be heattreated uniformly and effectively in vacuum; or in any inert(non-oxidizing) gaseous medium, such as, but not limited to, a helium ornitrogen “blanket”. However, heat transfer in a gaseous medium isgenerally not nearly as effective as heat transfer in an appropriatelyselected liquid medium. Consequently, during the heat treatment of largequantities of the particles of the invention (such as, but not limitedto, the output of a run of a commercial-scale batch production reactor),it is usually necessary to use a liquid medium, and furthermore also tostir the particles vigorously to ensure that the heat treatment isapplied as uniformly as possible. Serious quality problems may arise ifheat treatment is not applied uniformly; for example, as a result of theparticles that were initially near the heat source being overexposed toheat and thus damaged, while the particles that were initially far awayfrom the heat source are not exposed to sufficient heat and are thus notsufficiently postcured.

If a gaseous or a liquid heat treatment medium is used, the medium maycontain, without limitation, one or a mixture of any number of types ofconstituents of different molecular structure. However, in practice, themedium must be selected carefully to ensure that its molecules will notreact with the crosslinked polymer particles to a sufficient extent tocause significant oxidative and/or other types of chemical degradation.In this context, it must also be kept in mind that many types ofmolecules which do not react with a polymer at ambient temperature mayreact strongly with the polymer at elevated temperatures. The mostrelevant example in the present context is that oxygen itself does notreact with S-DVB copolymers at room temperature, while it causes severeoxidative degradation of S-DVB copolymers at elevated temperatures wherethere would not be much thermal degradation in its absence.

Furthermore, in considering the choice of medium for heat treatment, itis also important to keep in mind that the molecules constituting amolecular fluid can swell organic polymers, potentially causing“plasticization” and thus resulting in undesirable reductions of T_(g)and of the maximum possible use temperature. The magnitude of any suchdetrimental effect increases with increasing similarity between thechemical structures of the molecules in the heat treatment medium and ofthe polymer chains. For example, a heat transfer fluid consisting ofaromatic molecules will tend to swell a styrene-divinylbenzene copolymerparticle. The magnitude of this detrimental effect will increase withdecreasing relative amount of the crosslinking monomer (divinylbenzene)used in the formulation. For example, a styrene-divinylbenzene copolymerprepared from a formulation containing only 3% by weight ofdivinylbenzene will be far more susceptible to swelling in an aromaticliquid than a copolymer prepared from a formulation containing 35%divinylbenzene.

Geothermal gradients determine the temperature of the downholeenvironment. The temperature can be sufficiently high in some downholeenvironments to become effective in the postcuring of some compositionsof matter covered by the invention. Consequently, the “in situ”postcuring of the polymer particles, wherein the particles are placed inthe downhole environment of a hydrocarbon reservoir without heattreatment and the heat treatment then takes place in the environment asa result of the elevated temperature of the environment, is also withinthe scope of the invention.

It is important to note that the polymer particles are kept in thedownhole environment of a hydrocarbon reservoir for a very long time inmany applications. Consequently, temperatures which may be too low toprovide a reasonable cycle time in postcuring as a manufacturing stepmay often be adequate for the “in situ” postcuring of the particles inthe downhole environment during use. On the other hand, theimplementation of postcuring as a manufacturing step often has theadvantage of providing for better quality control and greater uniformityof particle properties. While each of these two approaches may hence bemore suitable than the other one for use in different situations, theyboth fall within the scope of the invention. Furthermore, theircombination by (a) applying a postcuring step during manufacture toadvance polymerization and network formation, followed by (b) the “insitu” completion of the postcuring in the downhole environment, is alsowithin the scope of the invention.

Various means known to those skilled in the art, including but notlimited to the stirring, flow and/or sonication of an assembly ofparticles being subjected to heat treatment, may also be optionally usedto enhance further the effectiveness of the heat treatment. The rate ofthermal equilibration under a given thermal gradient, possibly combinedwith the application of any such additional means, depends on manyfactors. These factors include, but are not limited to, the amount ofpolymer particles being heat treated simultaneously, the shapes andcertain key physical and transport properties of these particles, theshape of the vessel being used for heat treatment, the medium being usedfor heat treatment, whether external disturbances (such as stirring,flow and/or sonication) are being used to accelerate equilibration, andthe details of the heat exposure schedule. Simulations based on thesolution of the heat transfer equations may hence be used optionally tooptimize the heat treatment equipment and/or the heat exposure schedule.

Without reducing the generality of the invention, in its currentlypreferred embodiments, the thermoset polymer particles are placed in anunreactive gaseous environment with nitrogen as the preferred unreactivegas during heat treatment. Appropriately chosen equipment is used, alongwith simulations based on the solution of the heat transfer equations,to optimize the heat exposure schedule so that large batches ofparticles can undergo thermal exposure to an extent that is sufficientto accomplish the desired effects of the heat treatment without manyparticles undergoing detrimental overexposure. This embodiment of theheat treatment process works especially well (without adverse effectssuch as degradation that could occur if an oxidative gaseous environmentsuch as air were used and/or swelling that could occur if a liquidenvironment were used) in enhancing the curing of the thermoset polymer.It is, however, important to reemphasize the much broader scope of theinvention and the fact that the particular currently preferredembodiments summarized above constitute just a few among the vastvariety of possible qualitatively different classes of embodiments.

C. Applications

The obvious practical advantages [see a review by Edgeman (2004)] ofdeveloping the ability to use lightweight particles that possess almostneutral buoyancy relative to water have stimulated a considerable amountof work over the years. However, progress in this field of invention hasbeen very slow as a result of the many technical challenges that existto the successful development of cost-effective lightweight particlesthat possess sufficient stiffness, strength and heat resistance. Thepresent invention has resulted in the development of such stiff, strong,tough, heat resistant, and environmentally resistant ultralightweightparticles; and also of cost-effective processes for the fabrication ofthe particles. As a result, a broad range of potential applications canbe envisioned and are being pursued for the use of the thermoset polymerparticles of the invention in the construction, drilling, completionand/or fracture stimulation of oil and natural gas wells. Withoutreducing the generality of the invention, in its currently preferredembodiments, the specific applications that are already being evaluatedare as a proppant partial monolayer, a proppant pack, an integralcomponent of a gravel pack completion, a ball bearing, a solidlubricant, a drilling mud constituent, and/or a cement additive.

The use of assemblies of the particles as proppant partial monolayersand/or as proppant packs generally requires the particles to possesssignificant stiffness and strength under compressive deformation, heatresistance, and resistance to aggressive environments. Enhancements inthese properties result in the ability to use the particles as proppantsin hydrocarbon reservoirs that exert higher compressive loads and/orpossess higher temperatures.

The most commonly used measure of proppant performance is theconductivity of liquids and/or gases (depending on the type ofhydrocarbon reservoir) through packings of the particles. A minimumliquid conductivity of 100 mDft is often considered as a practicalthreshold for considering a packing to be useful in propping a fracturethat possesses a given closure stress at a given temperature. It is alsoa common practice in the industry to use the simulated environment of ahydrocarbon reservoir in evaluating the conductivities of packings ofparticles. The API RP 61 method is currently the commonly acceptedtesting standard for conductivity testing in the simulated environmentof a hydrocarbon reservoir. As of the date of this filing, however, workis underway to develop alternative testing standards.

It is also important to note that the current selection of preferredembodiments of the invention has resulted from our focus on applicationopportunities in the construction, drilling, completion and/or fracturestimulation of oil and natural gas wells. Many other applications canalso be envisioned for the compositions of matter that fall within thescope of thermoset polymer particles of the invention, extending farbeyond their uses by the oil and natural gas industry.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 shows the effects of advancing the curing reaction in a series ofisothermally polymerized styrene-divinylbenzene (S-DVB) copolymerscontaining different DVB weight fractions via heat treatment. Theresults of scans of S-DVB beads containing various weight fractions ofDVB (w_(DVB)), obtained by Differential Scanning calorimetry (DSC), andreported by Bicerano, et al. (1996), are compared. It is seen that theT_(g) of typical “as-polymerized” S-DVB copolymers, as measured by thefirst DSC scan, increased only slowly with increasing w_(DVB), andfurthermore that the rate of further increase of T_(g) slowed downdrastically for w_(DVB)>0.08. By contrast, in the second DSC scan(performed on S-DVB specimens whose curing had been driven much closerto completion as a result of the temperature ramp that had been appliedduring the first scan), T_(g) grew much more rapidly with w_(DVB) overthe entire range of up to w_(DVB)=0.2458 that was studied.

FIG. 2 provides an idealized schematic illustration, in the context ofthe resistance of thermoset polymer particles to compression as afunction of the temperature, of the most common benefits of using themethods of the present invention. In most cases, the densification ofthe crosslinked polymer network via post-polymerization heat treatmentwill have the main benefit of increasing the softening (and hence alsothe maximum possible use) temperature, along with improving theenvironmental resistance. In some instances, enhanced stiffness andstrength at temperatures that are significantly below the softeningtemperature may be additional benefits.

FIG. 3 provides a process flow diagram depicting the preparation of theexample. It contains four major blocks; depicting the preparation of theaqueous phase (Block A), the preparation of the organic phase (Block B),the mixing of these two phases followed by suspension polymerization(Block C), and the further process steps used after polymerization toobtain the “as-polymerized” and “heat-treated” samples of particles(Block D).

FIG. 4 shows the variation of the temperature with time duringpolymerization.

FIG. 5 shows the results of differential scanning calorimetry (DSC)scans. Sample AP manifests a large exothermic curing peak region insteadof a glass transition region when it is heated. Sample AP is, hence,partially (and in fact only quite poorly) cured. On the other hand,while the DSC curve of Sample IA20mG170C is too featureless for thesoftware to extract a precise glass transition temperature from it,there is no sign of an exothermic peak. Sample IA20mG170C is, hence,very well-cured. The DSC curves of Sample AP/406h6000 psi and SampleIA20mG170C/406h6000 psi, which were obtained by exposing Sample AP andSample IA20mG170C, respectively, to 406 hours of heat at a temperatureof 250° F. under a compressive stress of 6000 psi during the liquidconductivity experiments, are also shown. Note that the exothermic peakis missing in the DSC curve of Sample AP/406h6000 psi, demonstratingthat “in situ” postcuring via heat treatment under conditions simulatinga downhole environment has been achieved.

FIG. 6 provides a schematic illustration of the configuration of theconductivity cell.

FIG. 7 compares the measured liquid conductivities of packings ofparticles of 14/16 U.S. mesh size (diameters ranging from 1.19 mm to1.41 mm) from Sample IA20mG170C and Sample AP, at a coverage of 0.02lb/ft², under a closure stress of 5000 psi at a temperature of 220° F.,and under a closure stress of 6000 psi at a temperature of 250° F., asfunctions of the time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Because the invention will be understood better after further discussionof its currently preferred embodiments, further discussion of theembodiments will now be provided. It is understood that the discussionis being provided without reducing the generality of the invention,since persons skilled in the art can readily imagine many additionalembodiments that fall within the full scope of the invention as taughtin the SUMMARY OF THE INVENTION section.

A. Nature, Attributes and Applications of Currently PreferredEmbodiments

The currently preferred embodiments of the invention are lightweightthermoset polymer particles possessing high stiffness, strength,temperature resistance, and resistance to aggressive environments. Theseattributes, occurring in combination, make the particles especiallysuitable for use in many challenging applications in the construction,drilling, completion and/or fracture stimulation of oil and natural gaswells. The applications include the use of the particles as a proppantpartial monolayer, a proppant pack, an integral component of a gravelpack completion, a ball bearing, a solid lubricant, a drilling mudconstituent, and/or a cement additive.

In one embodiment, the polymeric particle has a substantially curedpolymer network; wherein a packing of the particles manifests a staticconductivity of at least 100 mDft after 200 hours at temperaturesgreater than 80° F. The particles are made by a method including thesteps of: forming a polymer by polymerizing a reactive mixturecontaining at least one of a monomer, an oligomer, or combinationsthereof. The at least one of a monomer, an oligomer, or combinationsthereof have three or more reactive functionalities capable of creatingcrosslinks between polymer chains. The particle is subjected to at leastone post-polymerizing process that advances the curing of a polymernetwork.

B. Compositions of Matter

The preferred embodiments of the particles of the invention consist ofterpolymers of styrene (S, non-crosslinking), ethyvinylbenzene (EVB,also non-crosslinking), and divinylbenzene (DVB, crosslinking).

The preference for such terpolymers instead of copolymers of S and DVBis a result of economic considerations. To summarize, DVB comes mixedwith EVB in the standard product grades of DVB, and the cost of DVBincreases rapidly with increasing purity in special grades of DVB. EVBis a non-crosslinking (difunctional) styrenic monomer. Its incorporationinto the thermoset polymer does not result in any significant changes inthe properties of the polymer, compared with the use of S as the solenon-crosslinking monomer. Consequently, it is far more cost-effective touse a standard (rather than purified) grade of DVB, thus resulting in aterpolymer where some of the repeat units originate from EVB.

The amount of DVB in the terpolymer ranges from 3% to 35% by weight ofthe starting mixture of the three reactive monomers (S, EVB and DVB)because different applications require different maximum possible usetemperatures. Even when purchased in standard product grades where it ismixed with a large weight fraction of EVB, DVB is more expensive than S.It is, hence, useful to develop different product grades where themaximum possible use temperature increases with increasing weightfraction of DVB. Customers can then purchase the grades of the particlesthat meet their specific application needs as cost-effectively aspossible.

C. Polymerization

Suspension polymerization is performed via rapid rate polymerization, astaught by Albright (U.S. Pat. No. 6,248,838) which is incorporatedherein by reference in its entirety, for the fabrication of theparticles. Rapid rate polymerization has the advantage, relative toconventional isothermal polymerization, of producing more physicalentanglements in thermoset polymers (in addition to the covalentcrosslinks).

The most important additional formulation component (besides thereactive monomers) that is used during polymerization is the initiator.The initiator may consist of one type molecule or a mixture of two ormore types of molecules that have the ability to function as initiators.Additional formulation components, such as catalysts, inhibitors,dispersants, stabilizers, rheology modifiers, buffers, antioxidants,defoamers, impact modifiers, plasticizers, pigments, flame retardants,smoke retardants, or mixtures thereof, may also be used when needed.Some of the additional formulation component(s) may become eitherpartially or completely incorporated into the particles in someembodiments of the invention.

D. Attainable Particle Sizes

Suspension polymerization produces substantially spherical polymerparticles. (While it is a goal of this invention to create sphericalparticles, it is understood that it is exceedingly difficult as well asunnecessary to obtain perfectly spherical particles. Therefore,particles with minor deviations from a perfectly spherical shape areconsidered perfectly spherical for the purposes of this disclosure.) Theparticles can be varied in size by means of a number of mechanicaland/or chemical methods that are well-known and well-practiced in theart of suspension polymerization. Particle diameters attainable by suchmeans range from submicron values up to several millimeters. Hence theparticles may be selectively manufactured over the entire range of sizesthat are of present interest and/or that may be of future interest forapplications in the oil and natural gas industry.

E. Optional Further Selection of Particles by Size

Optionally, after the completion of suspension polymerization, theparticles can be separated into fractions having narrower diameterranges by means of methods (such as, but not limited to, sievingtechniques) that are well-known and well-practiced in the art ofparticle separations. The narrower diameter ranges include, but are notlimited to, nearly monodisperse distributions. Optionally, assemblies ofparticles possessing bimodal or other types of special distributions, aswell as assemblies of particles whose diameter distributions followstatistical distributions such as gaussian or log-normal, can also beprepared.

The optional preparation of assemblies of particles having diameterdistributions of interest from any given “as polymerized” assembly ofparticles can be performed before or after the heat treatment of theparticles. Without reducing the generality of the invention, in thecurrently most preferred embodiments of the invention, any optionalpreparation of assemblies of particles having diameter distributions ofinterest from the product of a run of the pilot plant or productionplant reactor is performed after the completion of the heat treatment ofthe particles.

The particle diameters of current practical interest for various uses inthe construction, drilling, completion and/or fracture stimulation ofoil and natural gas wells range from 0.1 to 4 millimeters. The specificdiameter distribution that would be most effective under givencircumstances depends on the details of the subterranean environment inaddition to depending on the type of application. The diameterdistribution that would be most effective under given circumstances maybe narrow or broad, monomodal or bimodal, and may also have otherspecial features (such as following a certain statistical distributionfunction) depending on both the details of the subterranean environmentand the type of application.

F. Heat Treatment

The particles are placed in an unreactive gaseous environment withnitrogen as the preferred unreactive gas during heat treatment in thecurrently preferred embodiment of the invention. The inreactive gas thusserves as the heat treatment medium. This approach works especially well(without adverse effects such as degradation that could occur if anoxidative gaseous environment such as air were used and/or swelling thatcould occur if a liquid environment were used) in enhancing the curingof the particles.

Gases are much less effective than liquids as heat transfer media. Theuse of a gaseous rather than a liquid environment hence presentsengineering challenges to the heat treatment of very large batches ofparticles. However, these challenges to practical implementation areovercome by means of the proper choice of equipment and by the use ofsimulation methods.

Detailed and realistic simulations based on the solution of the heattransfer equations are hence often used optionally to optimize the heatexposure schedule. It has been found that such simulations becomeincreasingly useful with increasing quantity of particles that will beheat treated simultaneously. The reason is the finite rate of heattransfer. The finite rate results in slower and more difficultequilibration with increasing quantity of particles and hence makes itespecially important to be able to predict how to cure most of theparticles further uniformly and sufficiently without overexposing manyof the particles to heat.

In performing heat treatment as a manufacturing step as described above,which is the preferred embodiment of the invention, the usefultemperature range is from 120° C. to 250° C., inclusive. The duration ofthe exposure will, in practice, decrease with the maximum temperature ofexposure. More specifically, if the heat treatment temperature is 120°C., at least four hours of exposure to that temperature will berequired. On the other hand, if the heat treatment temperature is 250°C., the duration of exposure to that temperature will not exceed 20minutes. In the most preferred embodiments of the invention, theparticles undergo a total exposure to temperatures in the range of 150°C. to 200° C. for a duration of 10 minutes to 90 minutes, inclusive.

In other embodiments of the invention, where heat treatment is performed“in situ” in the downhole environment, the minimum downhole temperatureis 80° C. and the minimum dwell time in the downhole environment is oneweek. In practice, the minimum required amount of time for adequatepostcuring in the downhole environment will decrease with increasingtemperature of the environment. In more preferred embodiments of thisclass, the temperature of the downhole environment is at least 100° C.In the most preferred embodiments of this type, the temperature of thedownhole environment is at least 120° C.

EXAMPLE

The currently preferred embodiments of the invention will be understoodbetter in the context of a specific example. It is to be understood thatthe example is being provided without reducing the generality of theinvention. Persons skilled in the art can readily imagine manyadditional examples that fall within the scope of the currentlypreferred embodiments as taught in the DETAILED DESCRIPTION OF THEINVENTION section. Persons skilled in the art can, furthermore, alsoreadily imagine many alternative embodiments that fall within the fullscope of the invention as taught in the SUMMARY OF THE INVENTIONsection.

A. Summary

The thermoset matrix was prepared from a formulation containing 20% DVBby weight of the starting monomer mixture. The DVB had been purchased asa mixture where only 63% by weight consisted of DVB. The actualpolymerizable monomer mixture used in preparing the thermoset matrixconsisted of roughly 68.73% S, 11.27% EVB and 20% DVB by weight.

Suspension polymerization was performed in a pilot plant reactor, viarapid rate polymerization as taught by Albright (U.S. Pat. No.6,248,838) which is incorporated herein by reference in its entirety.The “single initiator” approach was utilized in applying this method.The “as-polymerized” particles obtained from this run of the pilot plantreactor (by removing some of the slurry and allowing it to dry atambient temperature) are designated as Sample AP.

Some other particles were then removed from the of the slurry, washed,spread very thin on a tray, and heat-treated for ten minutes at 170° C.in an oven under an unreactive gas (nitrogen) blanket. Theseheat-treated particles will be designated as Sample IA20mG170C.

FIG. 3 provides a process flow diagram depicting the preparation of theexample. It contains four major blocks; depicting the preparation of theaqueous phase (Block A), the preparation of the organic phase (Block B),the mixing of these two phases followed by suspension polymerization(Block C), and the further process steps used after polymerization toobtain the “as-polymerized” and “heat-treated” samples of particles(Block D).

Particles from each of the two samples were then sent to independenttesting laboratories. Differential scanning calorimetry (DSC) wasperformed on each sample by Impact Analytical, in Midland, Mich. Theliquid conductivities of packings of the particles of each sample weremeasured by FracTech Laboratories, in Surrey, United Kingdom.

The following subsections will provide further details on theformulation, preparation and testing of this working example, to enablepersons who are skilled in the art to reproduce the example.

B. Formulation

An aqueous phase and an organic phase must be prepared prior tosuspension polymerization. The aqueous phase and the organic phase,which were prepared in separate beakers and then used in the suspensionpolymerization of the particles of this example, are described below.

1. Aqueous Phase

The aqueous phase used in the suspension polymerization of the particlesof this example, as well as the procedure used to prepare the aqueousphase, are summarized in TABLE 1. TABLE 1. The aqueous phase wasprepared by adding Natrosol Plus 330 and gelatin (Bloom strength 250) towater, heating to 65° C. to disperse the Natrosol Plus 330 and thegelatin in the water, and then adding sodium nitrite and sodiumcarbonate. Its composition is listed below.

INGREDIENT WEIGHT (g) % Water 1493.04 98.55 Natrosol Plus 330(hydroxyethylcellulose) 7.03 0.46 Gelatin (Bloom strength 250) 3.51 0.23Sodium Nitrite (NaNO₂) 4.39 0.29 Sodium Carbonate (Na₂CO₃) 7.03 0.46Total Weight in Grams 1515.00 100.00

2. Organic Phase

The organic phase used in the suspension polymerization of the particlesof this example, as well as the procedure used to prepare the organicphase, are summarized in TABLE 2. TABLE 2. The organic phase wasprepared by placing the monomers and benzoyl peroxide (an initiator)together and agitating the resulting mixture for 15 minutes. Itscomposition is listed below. After taking the other components of the63% DVB mixture into account, the polymerizable monomer mixture actuallyconsisted of roughly 68.73% S, 11.27% EVB and 20% DVB by weight. Thetotal polymerizable monomer weight of was 1355.9 grams.

INGREDIENT WEIGHT (g) % Styrene (pure) 931.90 67.51 Divinylbenzene (63%DVB, 430.44 31.18 98.5% polymerizable monomers) Benzoyl peroxide (75%active) 18.089 1.31 Total Weight in Grams 1380.429 100.00C. Preparation of Particles from Formulation

Once the formulation is prepared, its aqueous and organic phases aremixed, polymerization is performed, and “as-polymerized” and“heat-treated” particles are obtained, as described below.

1. Mixing

The aqueous phase was added to the reactor at 65° C. The organic phasewas introduced 15 minutes later with agitation at the rate of 90 rpm.The mixture was held at 65° C. with stirring at the rate of 90 rpm for11 minutes, by which time proper dispersion had taken place asmanifested by the equilibration of the droplet size distribution.

2. Polymerization

The temperature was ramped from 65° C. to 78° C. in 10 minutes. It wasthen further ramped from 78° C. to 90° C. very slowly over 80 minutes.It was then held at 90° C. for one hour to provide most of theconversion of monomer to polymer, with benzoyl peroxide (half life ofone hour at 92° C.) as the initiator. The actual temperature wasmonitored throughout the process. The highest actual temperaturemeasured during the process (with the set point at 90° C.) was 93° C.The thermoset polymer particles were thus obtained in an aqueous slurrywhich was then cooled to 40° C. FIG. 4 shows the variation of thetemperature with time during polymerization.

3. “As-Polymerized” Particles

The “as-polymerized” sample obtained from the run of the pilot plantreactor described above will be designated as Sample AP. In order tocomplete the preparation of Sample AP, some of the aqueous slurry waspoured onto a 60 mesh (250 micron) sieve to remove the aqueous reactorfluid as well as any undesirable small particles that may have formedduring polymerization. The “as-polymerized” beads of larger than 250micron diameter obtained in this manner were then washed three timeswith warm (40° C. to 50° C.) water and allowed to dry at ambienttemperature. A small quantity from this sample was sent to ImpactAnalytical for DSC experiments.

Particles of 14/16 U.S. mesh size were isolated from Sample AP by someadditional sieving. This is a very narrow size distribution, with theparticle diameters ranging from 1.19 mm to 1.41 mm. This nearlymonodisperse assembly of particles was sent to FracTech Laboratories forthe measurement of the liquid conductivity of its packings.

After the completion of the liquid conductivity testing, the particlesused in the packing that was exposed to the most extreme conditions oftemperature and compressive stress were recovered and sent to ImpactAnalytical for DSC experiments probing the effects of the conditionsused during the conductivity experiments on the thermal properties ofthe particles.

4. “Heat-Treated” Particles Postcured in Nitrogen

The as-polymerized particles were removed from some of the slurry. Theseparticles were then poured onto a 60 mesh (250 micron) sieve to removethe aqueous reactor fluid as well as any undesirable small particlesthat may have formed during polymerization. The “as-polymerized” beadsof larger than 250 micron diameter obtained in this manner were thenwashed three times with warm (40° C. to 50° C.) water, spread very thinon a tray, and heat-treated isothermally for twenty minutes at 170° C.in an oven in an inert gas environment (nitrogen). The heat-treatedparticles that were obtained by using this procedure will be designatedas Sample IA20mG170C. A small quantity from this sample was sent toImpact Analytical for DSC experiments.

Particles of 14/16 U.S. mesh size were isolated from Sample IA20mG170Cby some additional sieving. This is a very narrow size distribution,with the particle diameters ranging from 1.19 mm to 1.41 mm. This nearlymonodisperse assembly of particles was sent to FracTech Laboratories forthe measurement of the liquid conductivity of its packings.

After the completion of the liquid conductivity testing, the particlesused in the packing that was exposed to the most extreme conditions oftemperature and compressive stress were recovered and sent to ImpactAnalytical for DSC experiments probing the effects of the conditionsused during the conductivity experiments on the thermal properties ofthe particles.

D. Differential Scanning Calorimetry

DSC experiments (ASTM E1356-03) were carried out by using a TAInstruments Q100 DSC with nitrogen flow of 50 mL/min through the samplecompartment. Roughly eight to ten milligrams of each sample were weighedinto an aluminum sample pan, the lid was crimped onto the pan, and thesample was then placed in the DSC instrument. The sample was thenscanned from 5° C. to 225° C. at a rate of 10° C. per minute. Theinstrument calibration was checked with NIST SRM 2232 indium. Dataanalysis was performed by using the TA Universal Analysis V4.1 software.

The DSC data are shown in FIG. 5. Sample AP manifests a large exothermiccuring peak region 510 instead of a glass transition region when it isheated. Sample AP is, hence, partially (and in fact only quite poorly)cured. On the other hand, while the DSC curve of Sample IA20mG170C 520is too featureless for the software to extract a precise glasstransition temperature from it, there is no sign of an exothermic peak.Sample IA20mG170C is, hence, very well-cured. The DSC curves of SampleAP/406h6000 psi 530 and Sample IA20mG170C/406h6000 psi 540, which wereobtained by exposing Sample AP and Sample IA20mG170C, respectively, to406 hours of heat at a temperature of 250° F. under a compressive stressof 6000 psi during the liquid conductivity experiments described below,are also shown. Note that the exothermic peak is missing in the DSCcurve of Sample AP/406h6000 psi, demonstrating that “in situ” postcuringvia heat treatment under conditions simulating a downhole environmenthas been achieved. For the purposes of this application the term“substantially cured” means the absence of an exothermic curing peak inthe DSC plot.

E. Liquid Conductivity Measurement

A fracture conductivity cell allows a particle packing to be subjectedto desired combinations of compressive stress (simulating the closurestress on a fracture in a downhole environment) and elevated temperatureover extended durations, while the flow of a fluid through the packingis measured. The flow capacity can be determined from differentialpressure measurements. The experimental setup is illustrated in FIG. 6.

Ohio sandstone, which has roughly a compressive elastic modulus of 4Mpsi and a permeability of 0.1 mD, was used as a representative type ofoutcrop rock. Wafers of thickness 9.5 mm were machined to 0.05 mmprecision and one rock was placed in the cell. The sample was split toensure that a representative sample is achieved in terms of its particlesize distribution and then weighed. The particles were placed in thecell and leveled. The top rock was then inserted. Heated steel platenswere used to provide the correct temperature simulation for the test. Athermocouple inserted in the middle port of the cell wall recorded thetemperature of the pack. The packings were brought up to the targetedtemperature gradually and equilibrated at that temperature.Consequently, many hours of exposure to elevated temperatures hadalready taken place by the inception of the collection of conductivitydata points, with the time at which the fully equilibrated cells wereobtained being taken as the time=zero reference. A servo-controlledloading ram provided the closure stress. The conductivity ofdeoxygenated silica-saturated 2% potassium chloride (KCl) brine of pH 7through the pack was measured.

The conductivity measurements were performed by using the followingprocedure:

-   1. A 70 mbar full range differential pressure transducer was    activated by closing the bypass valve and opening the low pressure    line valve.-   2. When the differential pressure appeared to be stable, a tared    volumetric cylinder was placed at the outlet and a stopwatch was    started.-   3. The output of the differential pressure transducer was fed to a    data logger 5-digit resolution multimeter which logs the output    every second during the measurement.-   4. Fluid was collected for 5 to 10 minutes, after which time the    flow rate was determined by weighing the collected effluent. The    mean value of the differential pressure was retrieved from the    multimeter together with the peak high and low values. If the    difference between the high and low values was greater than the 5%    of the mean, the data point was disregarded.-   5. The temperature was recorded from the inline thermocouple at the    start and at the end of the flow test period. If the temperature    variation was greater than 0.5° C., the test was disregarded. The    viscosity of the fluid was obtained from the measured temperature by    using viscosity tables. No pressure correction is made for brine at    100 psi. The density of brine at elevated temperature was obtained    from these tables.-   6. At least three permeability determinations were made at each    stage. The standard deviation of the determined permeabilities was    required to be less than 1% of the mean value for the test sequence    to be considered acceptable.-   7. At the end of the permeability testing, the widths of each of the    four corners of the cell were determined to 0.01 mm resolution by    using vernier calipers.    The test results are summarized in TABLE 3.    TABLE 3. Measurements on packings of 14/16 U.S. mesh size of Sample    AP and Sample IA20mG170C at a coverage of 0.02 lb/ft². The    conductivity of deoxygenated silica-saturated 2% potassium chloride    (KCl) brine of pH 7 through each sample was measured at a    temperature (T) of 190° F. (87.8° C.) under a compressive stress    (□_(c)) of 4000 psi (27.579 MPa), at a temperature of 220° F.    (104.4° C.) under a compressive stress of 5000 psi (34.474 MPa), and    at a temperature of 250° F. (121.1° C.) under a compressive stress    of 6000 psi (41.369 MPa). The time (t) is in hours. The liquid    conductivity (J) is in mDft.

T = 220° F., □_(c) = 5000 psi T = 250° F., □_(c) = 6000 psi J of J of tJ of AP IA20mG170C t J of AP IA20mG170C 29 558 669 22 232 225 61 523 64046 212 199 113 489 584 70 198 187 162 468 562 118 154 176 213 455 540182 142 159 259 444 527 230 137 147 325 418 501 264 135 145 407 390 477326 128 145 357 122 139 379 120 139 406 118 137

These results are shown in FIG. 7.

The liquid conductivity of the partial monolayer of the heat-treatedparticles under a closure stress of 5000 psi at a temperature of 220° F.is seen to be distinctly higher than that of the partial monolayer ofthe “as polymerized” particles that were postcured via “in situ” heattreatment in the conductivity cell at a temperature of only 220° F.

It is also seen that partial monolayers of both particles that wereheat-treated in a discrete additional post-polymerization process stepand “as polymerized” particles that were kept for a prolonged period inthe elevated temperature environment of the conductivity cell manifestuseful levels of liquid conductivity (above 100 mDft) even under aclosure stress of 6000 psi at a temperature of 250° F. The difference inliquid conductivity between the partial monolayers of these two types ofparticles is very small under a closure stress of 6000 psi at atemperature of 250° F., where long-term exposure to this rather hightemperature is highly effective in advancing the postcuring of the “aspolymerized” particles via “in situ” heat treatment as was shown in FIG.5.

The present disclosure may be embodied in other specific forms withoutdeparting from the spirit or essential attributes of the disclosure.Accordingly, reference should be made to the appended claims, ratherthan the foregoing specification, as indicating the scope of thedisclosure. Although the foregoing description is directed to thepreferred embodiments of the disclosure, it is noted that othervariations and modification will be apparent to those skilled in theart, and may be made without departing from the spirit or scope of thedisclosure.

1. A method for fracture stimulation of a subterranean formation havinga wellbore, comprising: injecting into the wellbore a slurry atsufficiently high rates and pressures such that said formation fails andfractures to accept said slurry; said slurry comprising a fluid and aproppant, wherein said proppant comprises astyrene-ethylvinylbenzene-divinylbenzene terpolymer composition having asubstantially cured polymer network, wherein said composition lacksrigid fillers or nanofillers, and emplacing said proppant within thefracture network in a packed mass or a partial monolayer of saidproppant within the fracture, which packed mass or partial monolayerprops open the fracture; thereby allowing produced gases, fluids, ormixtures thereof, to flow towards the wellbore wherein said packed massor partial monolayer of said proppant manifests a static conductivity ofat least 100 mDft after 200 hours at temperatures greater than 80° F.and under a given compressive stress of at least 5000 psi.
 2. The methodof claim 1, wherein said proppant has a shape; selected from the groupof shapes consisting of a powder, a pellet, a grain, a seed, a shortfiber, a rod, a cylinder, a platelet, a bead, a spheroid, or mixturesthereof.
 3. The method of claim 1, wherein a largest principal axisdimension of said proppant does not exceed 10 millimeters.
 4. The methodof claim 1, wherein said diameter ranges from 0.1 mm to 4 mm.
 5. Themethod of claim 1, wherein said proppant is a spherical bead having adiameter that does not exceed 10 millimeters.
 6. The method of claim 1,wherein said proppant is blended with other solid particles including atleast one of sand, resin-coated sand, ceramic and resin-coated ceramic.7. The method of claim 1, wherein said packed mass or partial monolayerof said proppant has an improved static conductivity compared to apacked mass or partial monolayer of proppant comprising astyrene-ethylvinylbenzene-divinylbenzene terpolymer composition, whereinsaid composition lacks rigid fillers or nanofillers and lacks asubstantially cured polymer network.